fix(ci): mount test/ dir excluded by .dockerignore into builder container

The .dockerignore excludes test/ so COPY . . in the builder stage never
includes it. Bind-mount the checked-out test/ directory into the container
at runtime so regression tests can find /build/test/regression.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: YuweiXiao

.github/workflows/ci.yaml

@@ -56,6 +56,7 @@ jobs:
         id: run-tests
         run: |
           docker run --rm \
+            -v ${{ github.workspace }}/test:/build/test \
             -v /tmp/regression-output-${{ matrix.pg_version }}:/results \
             pg_duckpipe:builder-${{ matrix.pg_version }} bash -c '
               set -euo pipefail